It is well known within air treatment technology to, in various ways, provide an air treatment device with some kind of heat recovery equipment to reduce heating costs for heating of cold outdoor air to warm supply air in a plant. Common variants are air based solutions, for example, the rotary heat exchanger and the cross flow heat exchanger. Furthermore, there are also liquid-coupled solutions with some form of liquid that circulates between the hot and the cold side for absorbing heat from the exhaust air and releasing heat to the supply air.
To further take advantage of the energy in the exhaust air, heat pump solutions (Heat Pumps) are used, wherein a so-called direct expansion coil—a DX-coil—is positioned in the supply air, in the air flow direction after heat recovery, and one DX-coil is positioned in the extract air, that is in the air after the heat exchanger, thus the air to be released into the free. In heating mode, the DX-coil positioned in the exhaust air constitutes a evaporator, while the DX-coil in the supply air constitutes a condenser. DX-coils usually consist of a number of copper tubes with aluminum fins, where the refrigerant passes through the copper tube and heat is emitted or absorbed through exchange with the ambient air passing through the DX-coil. The aluminum fins increases the heat transfer surface of the DX-coil. The heat pump process is not described in detail here. By the heat pump, more heat energy can be recovered from the exhaust air/extract air and by the heat pump process be transferred with higher energy content to the supply air. Also solutions that only use the heat pump as “recyclers” occur of course too.
Although heat recovery today usually is very effective—a rotating heat exchanger for example reaches a temperature efficiency of about 85%—there are in many cases a need of adding external heat energy to cope with the heating of the supply air during all times of the year, particularly in winter, and parts of fall and spring, of course depending on where the air handling plant is located geographically. Most common in Nordic countries, is to position a heating coil or a liquid-coupled heating coil, for example connected to a district heating system, in the supply air after the heat recovery device, to increase the supply air temperature to the desired level, in cases where the heat exchanger and/or heat pump is not capable of the full warming. In Europe, it is common to provide air treatment devices with a so called “frost-coil”, which is a heating coil for supply of external energy in order to defrost the heat exchanger. For that purpose, the heating coil is placed in the air flow direction before the heat exchanger, on the supply side, i.e. the external heat energy is supplied to the incoming outdoor/supply air in this position.
A known problem with heat pumps is that the operation becomes inefficient and in some cases with high wear on the heat pump components as a result, when the temperature of the air passing the evaporator gets too low, For Nordic climate it is common to recommend that the heat pump switches off at a temperature around −15° C. When the heat pump cannot be utilized, the entire heat pump contribution to the heat of the supply air has to be replaced by additional heating, an external heater is normally placed, as described above, in the supply air after the heat recovery equipment. At a heat pump the relationship between input driving energy and the recovered heat energy is described as the COP value, and typically, this value is between COP 2-5, meaning that the extracted energy is 2-5 times as great as the electrical energy input to the heat pump. If the corresponding energy should be added to the supply air by means of an electric heating coil, the cost is directly proportional to the power consumption. If instead a liquid-coupled heating coil is used, the corresponding energy must be produced in one way or another and be supplied to the heating coil. There are advantages to install an electric heating coil because it is a simple and relatively inexpensive installation, while a liquid-coupled solution is significantly more expensive than the electric heating coil installation. In return, the liquid-coupled solution would be preferable if the right conditions exist for production and connection to the heat source. Further disadvantages of existing solutions are that when the whole addition of heat should take place in the supply air, at the coldest temperatures, the physical dimensions of the auxiliary heater becomes great because it is a high power to manage. Furthermore, if the electric heating coil is used, even the main fuse for the plant can be affected when the plant has to cope with a substantial electrical power, which in itself increases the cost of both installation and that operating costs increase with higher fees for higher main fuse, all these drawbacks despite these operating conditions occur only during a few occasions or for a shorter period of total operating time of the plant.